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<title>Issue 1019: Make integral_constant objects useable in integral-constant-expressions</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++11">C++11</a> status.</em></p>
<h3 id="1019"><a href="lwg-defects.html#1019">1019</a>. Make <code>integral_constant</code> objects useable in integral-constant-expressions</h3>
<p><b>Section:</b> 21.3.4 <a href="https://wg21.link/meta.help">[meta.help]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-11 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.help">issues</a> in [meta.help].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>

<p><b>Addresses UK 205 [CD1]</b></p>

<p>
<code>integral_constant</code> objects should be usable in integral-constant-expressions.
The addition to the language of literal types and the enhanced rules for
constant expressions make this possible.
</p>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
We agree that the <code>static</code> data member
ought be declared <code>constexpr</code>,
but do not see a need for the proposed <code>operator value_type()</code>.
(A use case would be helpful.)
Move to Open.
</p></blockquote>

<p><i>[
2009-05-23 Alisdair adds:
]</i></p>


<blockquote>
<p>
The motivating case in my mind is that we can then use
<code>true_type</code> and <code>false_type</code> as integral Boolean expressions, for example inside
a <code>static_assert</code> declaration.  In that sense it is purely a matter of style.
</p>
<p>
Note that Boost has applied the non-explicit conversion operator for many
years as it has valuable properties for extension into other metaprogramming
libraries, such as MPL.  If additional rationale is desired I will poll the
Boost lists for why this extension was originally applied.  I would argue
that explicit conversion is more appropriate for 0x though.
</p>
</blockquote>

<p><i>[
2009-07-04 Howard adds:
]</i></p>


<blockquote>
<p>
Here's a use case which demonstrates the syntactic niceness which Alisdair describes:
</p>

<blockquote><pre>
#define requires(...) class = typename std::enable_if&lt;(__VA_ARGS__)&gt;::type

template &lt;class T, class U,
    requires(!is_lvalue_reference&lt;T&gt;() ||
              is_lvalue_reference&lt;T&gt;() &amp;&amp; is_lvalue_reference&lt;U&gt;()),
    requires(is_same&lt;typename base_type&lt;T&gt;::type,
                     typename base_type&lt;U&gt;::type&gt;)&gt;
inline
T&amp;&amp;
forward(U&amp;&amp; t)
{
    return static_cast&lt;T&amp;&amp;&gt;(t);
}
</pre></blockquote>
</blockquote>

<p><i>[
2009-07 post-Frankfurt:
]</i></p>


<blockquote><p>
Move to Tentatively Ready.
</p></blockquote>

<p><i>[
2009 Santa Cruz:
]</i></p>


<blockquote><p>
Moved to Ready for this meeting.
</p></blockquote>



<p id="res-1019"><b>Proposed resolution:</b></p>
<p>
Add to the <code>integral_constant</code> struct definition in 21.3.4 <a href="https://wg21.link/meta.help">[meta.help]</a>:
</p>

<blockquote><pre>
template &lt;class T, T v&gt;
struct integral_constant {
  static const<ins>expr</ins> T value = v;
  typedef T value_type;
  typedef integral_constant&lt;T,v&gt; type;
  <ins>constexpr operator value_type() { return value; }</ins>
};
</pre></blockquote>





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